DS10 - Défi des autres savoirs

MultiphAse rocks Deformation In SubductiOn zoNes: high-pressure experiments, modeling and stress records – MADISON

Submission summary

Subduction zones are extraordinarily complex regions of the earth, where two tectonic plates converge, one plunging under the other. They concentrate the largest part of the seismic and volcanic activity on earth, and represent a prominent feature of plate tectonics, one of the dynamic styles observed on planets of our solar system. The descending slabs constitute the cold, descending part of the earth mantle convection. Subduction zones are therefore a crucial object to understand the earth ‘engine’ at human and geological timescales. Modelling the earth evolution through geological times, understanding how subduction works on a regional and planet scale, its mechanics from earthquakes to the Myrs convection timescales, requires quantifying rocks mechanical properties, e.g. their viscosities. MADISON will bring novel observations and quantifications of deformation mechanisms for subduction zones rocks, from crystal to aggregate scale, by coupling three complementary approaches : experiments, numerical models and geological samples study.
The major sheared region at the interface between the subducting slab and the overlying mantle is critical. The degree of mechanical coupling, how much the plunging plate drags the warm adjacent mantle, has strong implications for the whole subduction dynamics. It influences the local convection and thermal fluxes of the subduction zone, and the depth where hydrated minerals in the subducting plate release fluids and induce mantle partial melting, and the associated volcanism. On shorter timescales, earthquakes location and magnitude depend on strength and location of stress “asperities” in the subduction zone.
Mineralogical composition has a large influence on rock viscosities and their deformation mechanisms. We will work on three major subduction zones rocks of the interface between the slab and the overlying mantle wedge that are suspected to play a strong role in its dynamics : the mantle peridotite transformed into a ‘serpentinized peridotite’ by fluids circulating, the former oceanic crust transformed in characteristic mineral assemblages called ‘blueschists’ and ‘eclogites’.
The minerals constituting the rocks have each different intrinsic plastic and elastic properties. Moreover, pressure and temperature increase during subduction modify these properties. MADISON’s specificities are i) to take into account these different mechanical properties when quantifying the deformation properties, while a common approach is to consider the most abundant mineral controls the deformation, and ii) to measure these properties at the relevant P-T conditions.
We will carry out high-pressure deformation of two-phases model samples for our target rocks. State-of-the-art deformation experiments using synchrotron x-ray in-situ, time-resolved measurements such as X-ray tomography and X-ray diffraction (XRD) on the samples under high-pressures and high-temperatures, will allow us to study with unprecedented time resolution the evolution of the microstructures and stresses/strains partitioning and deformation mechanisms in minerals as a function of mineralogical content, temperature, strain, macroscopic strain rate, aggregate flow stress, and pressures. Using emerging numerical techniques we will build a Crystal-Plasticity Finite Elements Model based on the experimental microstructures from tomographies. The model will reproduce the full field of deformation and stresses in our model rocks and will allow further understanding the physical processes at play. Microstrains recorded in natural rocks exhumed from subduction zones, and experimental samples will be analysed by conventional microstructural tools and new XRD techniques and cross-compared. By the end of the project, the integration of the results from the three approaches will allow unprecedented insights into the deformation processes of polymineralic rocks and on the nature of the record of the stresses in rocks.

Project coordination

Nadege Hilairet (Unité Matériaux et Transformations)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

UMET Unité Matériaux et Transformations

Help of the ANR 248,024 euros
Beginning and duration of the scientific project: December 2017 - 48 Months

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